No hot water, intermittent when air temp drops?

My vaillant turbomax 282e has an intermittent fault on the hot water side. On demand the sequence commences correctly but when it gets to the flame heat sensor final stage the boiler locks out with the red light appearing. New flame sensor fitted, problem persists. The common theme is when the air temperature drops overnight the lock out occurs on first demand in the morning. The micro switch sequence appears fine as does pressures etc. Any ideas most welcome